Hardness versus Wear resistance

The first post about the hardness of the aluminum oxide formed by the hard anodizing process described the various types of hardness test, Vickers, Knoops, Brinell and Rockwell.

This second one will give an idea of the differences between the hardness of the hard anodized coating (hard anodizing) and the wear resistance of the hard anodize anodic process. This text will also have referrals to presentations at the International Hardanodizing Association´s symposium in September 2010 in Las Vegas.

Aluminum oxide is almost as hard as diamond (1200 HV or more) but in general too thin to increase the hardness of the aluminum metal itself. It will not protect against strong pressure but it will resist surface scratches and therefore protect the overall appearance of the surface.

The hardness of the aluminum oxide layer formed by anodizing, increases by decrease in temperature of the electrolyte and in the acid concentration. The hardness is also increased by an increase in the homogeneity of the microstructure and by an increase in the current density used to form the aluminum oxide film.

Prof. Allan Matthews of the University of Sheffield, England pointed out in a presentation that "Wear = constant load/hardness is a commonly accepted relationship. However this equation is nonsense, because it ignores the many different types of wear, such as impact, fretting, abrasion, friction sliding and others."

In fact, the Elastic modulus is also very influential, and the ratio of hardness (H) to modulus (E) gives a better indication of wear resistance than either alone.

If the hardness is too high, the coating is susceptible to cracking. However, ductility allows a coating to accommodate deformation. When the ration H/E is high, then wear resistance is good.

Mr. Leonid Lerner from Sanford Process Corp., US showed in his presentation at the IHAA symposium a great slide of the two different directions which we expose the oxide layer for external stresses depending on if we test or use it in normal applications.

When testing the aluminum oxide film formed by the hard anodizing process it is normally done on a cross, shown in Image A and is explained more in the first post about how to define the hardness of aluminum oxide formed by hard anodizing.

This leads to a stress horizontal and perpendicular to the hexagonal oxide cell structure (Cross-sectional View, see right bottom of the slide).

Whereas the mechanical stresses in normal applications will be vertical and perpendicular to the hexagonal oxide cell structure (Top View, see left bottom of the slide).

So even though the hardness of the aluminum oxide film itself is very hard, it is way to thin to increase the hardness of the aluminum metal itself.

The hardness of the aluminum material is most often proportional to the abrasive wear resistanc but as explained above,the hardness of the aluminum oxide film formed by hard anodizing will not always be proportional to abrasive wear resistance.

Maximum abrasion resistance of the aluminum oxide is found on pure aluminum and aluminum-magnesium alloys for the same hard anodizing process parameters.

Sealing decreases the wear resistance of oxide film formed by hard anodizing up to 50 - 70 % of the unsealed value.

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